The answer to this is just simple common sense. Just compare the two interpretations. But I want to point out a few things. First, It should be pointed out that even if you had like 10 or say 20 kids would you really have subdued the Earth? Its a bit silly to think so. Second, I would like to point out that abortion and the birth control pill ARE evil because the pill can be abortive in nature and abortion is just sick. (don't get it confused)-------------------- Now comparing the two interpretations using common sense: My interpretation is that God isn't stipulating multiplication as a commandment (now the Vatican on the other hand apparently has), but it was meant more as reassurance that procreating with a spouse is fine and viewed as more good than it is evil. And may even be blessed where as to be completely good. I mean if that interpretation doesn't make sense to you let's examine the other side, the other interpretation in which that God does DICTATE for us to procreate AND usually which has gone a step further even to say that EVERY time you be "with" your spouse you MUST engage in "that certain specific act" which could result in creating a new life. I don't read that level of specificity when God is saying "Have many children." especially when elsewhere the Bible states that it is GOOD if a man never even touches a woman at all. (First Corinthians 7:1 ) What? is God the marital sex police to tell us how sex is to end between two married people? If you're still not following or you are mad at what I'm saying consider the following. For instance if you go to a friend's house and the friend says: Here, drink this tea. Is the the friend commanding you to drink the tea just because it was stated? or simply suggesting it ( which by the way I am 110% sure God is more than capable of suggesting something )? How else would God word a suggestion? "I suggest to" or something weird like that? Is the second interpretation trying to say that God is incapable of a suggestion and is merely only capable of commandments? A suggestion is simply the case we have here, not all this about thou shall not pull-out and similar nonsense.

Source: The Bible ( First Corinthians 7:1 )

This is a somewhat more radical answer and I don't know of any established sects that have it as their view, but one theory I've had, which I don't hold as truth, but only a possibility, is that Adam and Eve may have been the first people to have souls and that they needed to multiply to spread this characteristic throughout humanity.

This is a very unconventional view and why I don't view it as a certainty, but only a theory, but when Cain was sent away, it indicates there were people there and that he was able to find a wife. There are also references to the sons of man and the sons of God intermarrying which brings up a question of where those other people came from since they are not mentioned directly.

I actually came up with this theory as one of several possible ways that a literal reading of Genesis could match with what science believes about evolution of life on Earth. If this theory is accurate, then the command to multiply would serve to spread human spirit throughout the population. We clearly don't see anyone now that doesn't have a spirit since the only survivors of the flood would have had it.

Again though, I have to stress that this is just a theory and should not be taken as doctrine. Ultimately, the reason for the command isn't doctrinally significant. As others have pointed out, it was never repeated to anyone else and the general reason that most theories agree on was that it was to start the species, either to populate the planet or to introduce spirit and free-will.

As to why this would be a good thing, ultimately, God's plan has always been to bring anyone who is willing to come back to him back. Without giving someone that opportunity, they can't make that choice. The opportunity comes with costs of it's own (living in a fallen world), but isn't a life in this world worth the opportunity to choose to follow God?

Possibly God commanded us a His followers to procreate so that we will begin something here on earth, that will echo in eternity on the New Earth! Our believing children will populate the New Heaven and New Earth! Isn't that reason enough? What greater example of reaping and sowing throughout eternity than children to bask in the Glory of God forever?

Like all of God's other commandments, the commandment has multiple purposes:

Immediate Result

The immediate result of the commandment is that the earth gets populated, and God says so as part of the commandment.

Babies are Good

Another reason for every commandment is to emphasize how we should think about things. This commandment tell us that babies are good.

You are not the Whole Purpose of Your Existence

God did not put you on the earth so that you could while away your life pursing your own pleasures and interests and leave nothing behind when you are gone.

(As far as non-Christians having children, of course God wants that, too. After all, this commandment was originally given to non-Christians.)

Why?

We may never know. Ps. 103:7

"Why" would mean your looking for reason. This asks the reason God does things; thereby a peek into the grand scheme of his plans, an understanding into God's innate motivation, ultimately… inferring into the very mind of God.

We are yet to fully understand the human mind, let's leave the mind of God for a later day (:

So what is the reason God wants us to have children?

Why? Because he loves us so much. If I were to say it the way I'd really want to, I'd say:

God wants us to have become children because he really, really loves us.

In other words, God wants us to become living, breathing humans because he loves us so much. And his way of making humans, naturally, is as the human children of human moms and dads.

God wants us to have children so that he can bring people into the world, and these are not just generic human beings stamped out from the presses of heaven. These people are you and me. These people are people that he knew long before he ever formed them in their mother's womb (Jer. 1:16)...people whose every day he prepared in advance (Ps. 139:16) so that we could know him (Phil. 3:10, Eph. 2:10), walk in him (Col. 2:6), live and move and have our being in him (Acts 17:28), and completely find our identity in him (Col. 2:20). Giving us the opportunity to live life on this Earth gives us a unique opportunity that is only possible this side of heaven. That opportunity is to walk by faith and not by sight (2 Cor. 5:7, Jn. 20:29). Can this kind of opportunity ever really exist again once we get to heaven? For there we will walk by faith and sight, as we will evermore behold him face to face (1 Jn. 3:2, Rev. 22:4, 1 Cor. 13:12).

We know how important faith without sight is to the Lord:

Now faith is the assurance of things hoped for, the conviction of things not seen. For by it the people of old received their commendation. By faith we understand that the universe was created by the word of God, so that what is seen was not made out of things that are visible. By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks. By faith Enoch was taken up so that he should not see death, and he was not found, because God had taken him. Now before he was taken he was commended as having pleased God. And without faith it is impossible to please him, for whoever would draw near to God must believe that he exists and that he rewards those who seek him. (Heb. 11:1-6)

How deeply pleasing it is to him when we give our lives completely to him even though we can not see him with our earthly eyes. During our lives here on earth, we have the only opportunity we will ever have to walk by faith without sight. So then, what an immensely precious thing it is to God that we have this earth-life now before eternity! How precious it is to him that our love for him be proven real in the scope of a world where we can only apprehend him by the spirit...where the physical eye and ear will likely never behold his form this side of heaven.

For you formed my inward parts; you knitted me together in my mother's womb. I praise you, for I am fearfully and wonderfully made. Wonderful are your works; my soul knows it very well. My frame was not hidden from you, when I was being made in secret, intricately woven in the depths of the earth. Your eyes saw my unformed substance; in your book were written, every one of them, the days that were formed for me, when as yet there was none of them. (Ps. 139:13-16)

So, in summary, God want us to have children because there are many more people who have not yet been created that he already knows. He wants them to be given human form so that they would have the chance to know him likewise. And in particular, he wants them to know him from the vantage point of an earthly existence, where faith without sight--something so difficult to have in a world where the earthly senses rule--proves real the love of a man toward God. God loves us so much, and living life on earth gives us a unique opportunity amidst all time, space, and eternity to walk by faith and not by sight.

He did in the Old Testament, but that precept no longer applies today because the world is sufficiently populated.

Jesus Himself in the New Testament nowhere commands people to have children. In fact, the divinely inspired St. Paul in 1 Cor. 7 says virginity is preferable to marriage.

In light of a suggestion I may want to edit this answer to make the connection to the OP clearer, I am submitting this reworked version.

The question being discussed is “So what is the reason God wants us to have children?” So far several theories have been submitted discussing various permutations of God’s command to Adam and Eve to “Go forth and multiply”, why that command was given and if it still bears true for us today.

While this is an SE dealing with hermeneutics, sometimes the best answer to a question does not come through proper exegesis, but through practical application. Therefore, I would offer a different approach to this in that the command for the having of children is not so much as to increase population, although that is the primary reason implicit in the command, as it is to serve as an object lesson teaching us some of the nature of God and our relationship with Him.

It is through having children that God may show us many truths that, while we can gain some level of understanding through reading them, when personally experienced open our eyes more so than simply reading about them ever could. For instance, through the role of parenting, He might help us to understand better that God does not put laws into place to restrict our action but to protect us from harm and therefore give us more freedom than we would have ever had before. He will show us a more direct example of the grace and patience He extends to us out of love. We should hopefully understand better the willingness to sacrifice anything and everything for the well-being of our child. Though we understand God sometimes answers our prayers “no”, we can better understand why God says "No" to what we think are perfectly reasonable requests but He knows is not the best thing for us. He can also help us to realize why He blesses us at times we have done nothing to deserve such blessings.

In addition to learning about being a parent, we can also see our roles as children of a perfect father. Not that any of us are perfect parents, but through our own failings we can understand God better. For instance, when we lose patience with a child doing something they know not to do, God says "Now you know how I feel." We get a better grasp of God’s frustration when we rebel against him as our teens do to us. Parenting helps us to know on a personal level how God loves it when we spontaneously tell Him we love Him, not because we are responding to a blessing, but just because we love Him. Maybe we see better how frustrated God gets when we fight and argue and get selfish with others -- especially those in our own family of Christians. But we can also realize how He looks at us with pride when we are kind and helpful. Finally, through being a parent and helping our own children learn to walk, we can learn how He does not look at our stumbles as we are trying to walk with anger and impatience but celebrates each wobbly step we take and when we fall on our (spiritual) diaper, He picks us back up, hugs us, and helps us to take another step.

I suggest God commanded us to have children because He loves us. Being a parent is a practical way of giving us a glimpse just how much. He wants us to be parents because He is a parent. He wants us to have children because we are His children.

The short answer is that, according to the Bible, Christians are neither obligated or encouraged to have children. What follows is a more in-depth, Bible based answer.

Genesis 1:28 was a command given to Adam and Eve, the first humans, not to Christians. Why did got want them to produce offspring? Well, to put it simply, if they did not, they would have been the only two humans on the earth. The purpose of the earth is to support human life and be our home. (Psalm 115:16) You have to remember that at the time God said this, Satan was not yet the ruler of the world. It was and still is God's purpose that the earth be filled with happy, healthy people. (Psalm 37:29; Psalm 104:5)

The Christian Greek Scriptures do not repeat the command to produce offspring as being binding on Christians. (Acts 15:28, 29) They also do not forbid getting married or having children. (1 Timothy 4:1-3) While singleness is never a requirement for Christians, the apostle Paul does discuss the benefits of singleness, provided we are not under excessive pressure, not "inflamed with passion" and therefore in danger of committing fornication. The course of singleness is "better" in that it allows one to serve God without distraction. (1 Corinthians 7:1, 2, 8, 9, 29-38) In reference to singleness, Jesus said "Let the one who can make room for it make room for it." (Matthew 19:10-12) Since the Bible recommends singleness for Christians under certain circumstances, we can safely say it is not encouraging us to have more children. Ultimately however, the decision to remain single, or get married and not have children, or to get married and have children is left up to a personal decision on our part. We should weigh seriously the effects and responsibilities that accompany these decisions. Even so, none of those options is considered a sin and we are permitted to choose what we want as long as we do not commit fornication or adultery.

This is what Jehovah's Witnesses believe. For more information, you can visit jw.org

Also, much of this information comes from the section on Singleness in Insight on the Scriptures.

Genesis 1:28 (NIV) God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish in the sea and the birds in the sky and over every living creature that moves on the ground.”

This was a blessing from God. Most of the other Bible translations also use the word "Be fruitful" and it indicates blessing. God blessed them and at the same time commanded them to rule the earth. "Rule the earth" is the first commandment we find in the Bible. Whether "Be fruitful" is a blessing or a commandment is up to individual's interpretation. For the sake of argument, let's say it was a command.

Does this (command) "Be fruitful" still stand?

The Old Testament laws are always a problem to Christians. Though we all agree that animal sacrifice is no longer required, we have arguments on the rest of the laws in OT. Many Christians are still following OT laws for Sabbath, unclean animals, eating blood etc. My view is that we should focus more on New Testament and it has very few laws. Even the Law of Moses did not repeat this command. Therefore, it might have less importance. The NT never encourages to have many children nor discourage it. Remember, Saint Paul was never married. And if this command still stands, the Catholics will be the first to disobey the law by introducing unmarried man and women in the services of the Church.

So what is the reason God wants us (or Adam and Eve) to have children?

This phrase "Be fruitful" was spoken to Adam and Eve, and it was never repeated later, not by Moses nor in the New Testament. So, initially God wanted Adam and Eve to reproduce and have many children and God designed them for that. It was the will of God that we should fill the earth. Now that we have filled the earth (or even overpopulated it), God's words are accomplished. What ever words that God has spoken are always accomplished.

Isaiah 55:11 (NIV) so is my word that goes out from my mouth: It will not return to me empty, but will accomplish what I desire and achieve the purpose for which I sent it.

Paul focused more on Evangelism than giving birth to more Christians: Saint Paul had his own personal views towards marriage and it seems he was not concerned about marriage and having children. He suggested that unmarried life is better for engaging in the Lord's affairs and married people cannot serve the Lord as efficiently as the unmarried ones.

1 Corinthians 7

8 Now to the unmarried and the widows I say: It is good for them to stay unmarried, as I do.

32-34 I would like you to be free from concern. An unmarried man is concerned about the Lord’s affairs—how he can please the Lord. But a married man is concerned about the affairs of this world—how he can please his wife— and his interests are divided. An unmarried woman or virgin is concerned about the Lord’s affairs: Her aim is to be devoted to the Lord in both body and spirit. But a married woman is concerned about the affairs of this world—how she can please her husband.

38 So then, he who marries the virgin does right, but he who does not marry her does better.

The Harvest is ready: God created a couple and blessed them to be fruitful. Our great population proves our fruitfulness. Now is the time to harvest the souls. God had in mind to bring as many souls as possible to Heaven. Now there are many souls for the harvest but the workers are few.

Luke 10:2 (NIV) He told them, “The harvest is plentiful, but the workers are few. Ask the Lord of the harvest, therefore, to send out workers into his harvest field.

The first thing I will note is an issue with the question. It is not fair to ask "Why does God ..." because it is wholly unanswerable unless the Scripture says explicitly why, but it usually does not explain God's actions. So I will focus more on "Why do some Christians believe ..." as there is usually an abundance of text where they explain their thoughts.

In today's modern society the question of whether to procreate or not is hidden under the question of whether contraception is moral or not. Yes, to some groups, using contraception is morally offensive, however, to the larger majority, it is more of a concern for the perpetuation of the faith. Invariably, allowing contraception leads to fewer births, which in turn leads to relatively fewer believers for the next generation. This concern is legitimate; the common phrase among those being candid about the issue is "out-breed the non-believers". The phrase is not unique to Christians either. Islamic clerics have said it on more than one occasion [Source]. It is often said and employed by racist groups as well, wanting to "out-breed" the other races¹. Even some atheists are worried they will be "out-bred" by the religious [Source].

So what does all this have to do with your question? Well, to answer it effectively for a modern audience, an examination on the various views on contraception will be very illuminating. There are a few categories (According to this Wikipedia source):

1. Children in abundance - Often members of the Quiverfull Movement, those in this group believe that any contraception, even Natural Family Planning (as defined by the RCC) is morally unacceptable. They believe that the use of birth control undoes God's purpose (or one of them) for marriage.

2. Children in managed abundance - Those is this group stress the importance of having many children, however, they are more open to the outcome. Generally, you are to have however many you can conceive, but Natural Family Planning is acceptable. The Catholics are very similar to this position.

3. Children in moderation - Those in this group encourage families to have children, however, they place a higher stress on exercising prudence when family planning. As an effect, they approve of the use of artificial contraception. Generally, the number of children you have is your choice.

4. No children - Those in this group believe they may choose to live their lives without consideration or obligation to having children. There really are not many denominations that fall into this category. The nearly defunct Shakers believe procreation is immoral.

Because your question assumes that God commands Christians to procreate I will only discuss the theologies of the first three.

For the "children in abundance" group, naturally, they often quote the Genesis command to Adam and Eve, and also the repeat of that command to Noah. More commonly is that they quote

Pslams 127:3 (NASB)
Behold, children are a gift of the Lord,
     The fruit of the womb is a reward.

They argue that if children are a gift from God, then it is greatly disrespectful to refuse those gifts. You would not refuse a gift of wealth from God; likewise a gift of health and long life. So, they argue, you should not refuse a gift of children. In addition to this, they also point to a large number of verses using the phrase "open and close womb" to support that God is in control of childbearing. Those verses are: Genesis 20:18 Genesis 29:31 Genesis 30:22 Samuel 1:5-6 and Isaiah 66:9. To answer "Why does God command ..." is clear. It is because He wants to bless us and give us the precious gifts of children.

Now, it seems to me, that the "children in managed abundance" and "children in moderation" groups are very similar to the first group. They will certainly agree that children are a gift and that God does, indeed, control the miracle of conception, but they consider at least this one thing: care for the children. The use of the word "prudence" by some in the third group implies that the welfare of the child should be considered when family planning, and there are circumstances when a new child under your care would put unnecessary hardship on the parents, the child, or both. The difference between groups two and three just seems to be the degree to which "prudence" is applied. However, I would note that the cultural "out-breed the non-believer" stance may be a factor in group two's beliefs, but likely not in group three's.

How would groups two and three answer "Why does God command ...?" Group two may reference the blessing verse, but group three certainly would not. In fact, what is more likely is that both groups would not necessarily agree that it is a command for all mankind. Some would even likely quote Paul saying "It is good for a man not to marry," (See 1 Corinthians 7 (NIV)) pointing out that neither him, nor Jesus married nor sired any children.

Wikipedia (again) has a section discussing denominational positions on contraception.

1. An anecdote. My wife worked with a man that admitted freely he believed in white racial supremacy and his church (likely very small) taught that they must "out-breed" the three M's: Mormons, Muslims, and Mexicans. Clearly, the first two could be taken without racism implied, however, not so much for the third.

God told Adam and Eve this as part of His blessing to them.

"And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth..." (Genesis 1:28)

Children are a blessing to the parents, and also are part of the pleasure that God receives from His creation. "Thou art worthy, O Lord, to receive glory and honour and power: for Thou hast created all things, and for Thy pleasure they are and were created." (Revelation 4:11)

After the flood, God knew that the survivors may have been reluctant to have large families, after having seen the mass destruction of so many people, so He encouraged them repeatedly in Genesis 8 and 9 to have the confidence to again replenish the earth.

It was in this context that God's 1st covenant with man was given to Noah, (promising that the earth would not be destroyed) as a further reassurance that they should "Be fruitful, and multiply, and replenish the earth". Genesis 9:1 [the first covenant was given in Genesis 9:9-17]

The context of the second covenant, given to Abraham (or Abram), also shows God's mind as to children in the light of being a blessing. "Look now toward heaven, and tell the stars, if thou be able to number them: and He said unto him, So shall thy seed be." (Genesis 15:5) [the 2nd covenant was given in verse 18]

God stresses this point to the Children of Israel, telling them that having children was part of His blessing to them. See, for instance Deuteronomy 7:13 "Thou shalt be blessed above all people: there shall not be male or female barren among you."

In the New Testament we find that the act of raising children was considered a merit. For instance widows were to be cared for if she were "well reported of for good works; if she have brought up children..." (1 Timothy 5:10), and the younger women (in verse fourteen) "that the younger women marry, bear children, guide the house..."

As for the men, children were also listed in the description of what traits were to be seen in an "elder" and a "deacon" in 1 Timothy 3.

Certainly an added blessing for Christians is to see their children "grow in grace, and in the knowledge of our Lord and Savior". As John wrote to "the elect lady, and her children" in his second epistle, "I rejoiced greatly that I found of thy children walking in truth, as we have received a commandment from the Father." (See verses 1 & 4)

"Lo, children are an heritage of the Lord: and the fruit of the womb is His reward." (Psalm 127:3)

The fruitfulness of the creation is something God intended according to the design of the world. He made the world a certain size intending that it become full of various things including vegetation, water, animals and man. To be fruitful is not a command as though fishes that did not propagate were sinful fishes, it is more of an 'ordinance of blessing' and thus a declared 'law of nature'. Yes the word of God says to the fish ‘Be fruitful and increase in number and fill the water in the sea’ (Ge 1:22) but this is simply a confirmation of his goodwill and design confirmed upon the fish. This is naturally not understood as an actual proscription against a fish negelecting procreation. The same applies to human propagation.

Various commentators on Genesis have recognized this basic feature of the intended fruitfulness of God’s creation according to his own infinitely wise design. For example in Luther we find this argument:

For this word which God speaks, “Be fruitful and multiply,” is not a command. It is more than a command, namely, a divine ordinance [werck] which it is not our prerogative to hinder or ignore. Rather, it is just as necessary as the fact that I am a man, and more necessary than sleeping and waking, eating and drinking, and emptying the bowels and bladder. It is a nature and disposition just as innate as the organs involved in it. Therefore, just as God does not command anyone to be a man or a woman but creates them the way they have to be, so he does not command them to multiply but creates them so that they have to multiply. And wherever men try to resist this, it remains irresistible nonetheless and goes its way through fornication, adultery, and secret sins, for this is a matter of nature and not of choice. (Luther, M. (1999). Luther’s works, vol. 45 : The Christian in Society II. (J. J. Pelikan, H. C. Oswald, & H. T. Lehmann, Eds., Vol. 45, p. 18)

Again Philip Henry (Matthew Henry’s father) wrote:

Be fruitful and multiply. This is not a command, of which they are transgressors that never marry, or that have no children; but a promise that they should be fruitful.( Henry, P. (1839). Exposition of the First Eleven Chapters of Genesis (p. 240). London: J. Nisbet and Co.)